Our goal is the development of a clinically applicable, non-viral, muscle-directed gene therapy for Crigler- Najjar Syndrome Type I (CN-I). Due to mutations in the bilirubin UDP-glucuronosyltransferase (UGT1A1) gene, CN-I patients cannot excrete bilirubin. The resulting hyperbilirubinemia causes toxic brain damage. Treatment modalities are aimed at decreasing serum bilirubin levels to prevent brain damage. Liver transplantation is the only approach that leads to long-term metabolic correction; all other known medical measures are palliative at best. Despite considerable efforts, an optimal gene therapy method is also lacking. We propose using genetically modified skeletal muscle to bypass the block in hepatic bilirubin metabolism. Recent improvements in naked plasmid DMA (pDNA) mediated gene transfer into muscle have made this approach clinically viable. Simple intravenous pDNA injection can achieve highly efficient gene expression throughout the limb muscles in a variety of animals, including monkeys, predicting that this procedure will be useful in humans as well. Our preliminary data in the UGT1A1 deficient Gunn rat, the animal model of CN-I, prove the principle of the muscle directed approach: intravenous injection of naked plasmid DNA (pDNA) expressing human UGT1A1 enabled uptake, glucuronidation and release of bilirubin by muscle, leading to biliary excretion of bilirubin and amelioration of hyperbilirubinemia. However, we found that stability of gene expression is limited by immune responses against the (human) UGT1A1 protein. In this proposal we will investigate whether this immune response can be prevented, and stability of expression improved, by expressing the rat UGT1A1 gene driven by muscle-specific promoters. Metabolic correction will be followed by serum bilirubin measurements. Bilirubin glucuronidation will be confirmed by HPLC analysis of bile. effects of gene therapy on chronic bilirubin toxicity (renal tubular damage) will be studied. These studies will provide proof-of-concept data for this gene therapy approach.